Yes Larger organisations are necessary like in HOI disvisions are grouped into larger corps and armies. It's just not practical and ahistorical to move individual regiments around. The point is what the basic military unit represented in the game should be?
HOI enables you to handle armies of several hundred divisions with several thousand units in total. Thus for a XIXth century game, divisions could be replaced by regiments, though numbers would swell from 1880 onwards.
For exemple in 1870 France would have around 200 regiments overall (including infantry cavalry guards....) . This number had remained roughly stable for about 250 years, with some variations such as the Napoleonic wars and various reorganisations.
With the introduction of conscription, that number swelled.
An other point is that raising new armies from scratch is a lengthy process. New units can be formed in a few weeks but it takes months before they can be trained properly if experienced cadres are lacking. This played a critical role in several conflicts.
American civil war: both sides started with inexperienced forces and it took them some time before they got experienced.
1870, France lost its regular army within a month and though they raised masses of new troops they were no match for the Germans
1914, It took a lot of time for the British to raise their New Armies and train them, though they did not reach the level of efficiency of the old professional army.
This periode sees the transition from small standing armies to mass conscription armies and the introduction of peace time military service for all.
